The human eye can process an astonishing dynamic range from snow in bright sunlight, to the light of a glow worm in a dark cave. Photographers aim to capture the widest possible range of brightness values of objects in a scene. Previously, this is achieved by manually dodging and burning dark and light areas of the photograph respectively. With the advent of digital photography the problem of displaying high dynamic range image data on low dynamic range media, such as paper or monitors, has emerged.
FIG. 1(a) illustrates a scene 102 comprising the sun 104 shining through a window 106 into a room 108. The window 106 is a stained glass window with a picture 110 on it. An object 112 is located on a wall next to the window such that the object is not directly illuminated by the sun 104. As a result, there is a great difference in brightness between the picture 110 in the window 106 and the object 112. A high dynamic range camera 114 captures an image of the scene 102 as a digital high dynamic range photograph. The photograph is then displayed on low dynamic range display 116.
FIG. 2 illustrates the high dynamic range 210 of camera 114 and the low dynamic range 120 of display 116. The image values, such as pixel intensities, of the bright window 106 span an upper part 212 of the high dynamic range 210 while the image values of the dark object 112 span a lower part 214 of the high dynamic range 210.
In order to display the high dynamic range image from camera 114 on the low dynamic range display 116, the image values of the high dynamic range 210 are mapped onto the low dynamic range 220, which is commonly referred to as tone mapping.
In cases where the bright window 106 is the most important subject of the image, the upper part 212 of the high dynamic range 210 is stretched to span the entire low dynamic range 220. Image values that are below the lower end of the upper part 212 are mapped to black. The result of such a tone mapping is illustrated in FIG. 1(b) which shows that in the dynamic range the window 106 is clearly visible, but the dark object is not distinguishable from the black wall.
In other cases where the dark object 112 is the most important subject of the image, the lower part 214 of the high dynamic range 210 is stretched to span the entire low dynamic range 220. Image values that are above the upper end of the lower part 214 are mapped to white. The result of such a tone mapping is illustrated in FIG. 1(c) where the object 112 is clearly visible, but the picture is not distinguishable from the white window 106.
Neither of the results of FIG. 1(b) and FIG. 1(c) are desirable if both the picture 110 on window 106 as well as the object 112 are to be visualised. A more sophisticated method is to stretch the upper part 212 and the lower part 214 by a different factor and thereby utilise the low dynamic range optimally. This would result in a piecewise linear mapping of the high dynamic range image data 210 to the low dynamic range 220. This approach may be further extended to non-linear mapping. However, with any stretching of one part, another part is inevitably compressed
Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each claim of this application.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.